Lateral flow immunoassay (LFIA) technology has seen commercial success in recent years because of its simplicity, low cost and user friendliness. The technology is particularly suitable for point-of-care (POC) test markets. It has been utilized for detection of a large number of analytes from small molecules, enzymes, macromolecules to microorganisms. One or several analytes can be tested for simultaneously on the same strip. Urine, saliva, serum, plasma, or whole blood can be used as specimens. Extracts of patient exudates or fluids have also been successfully developed. Lateral flow tests, or immunochromatographic strips, were first introduced as qualitative urine pregnancy tests used in doctors' offices and at home. They have evolved into rapid tests for a range of analytes, including HIV, respiratory diseases, drugs-of-abuse, cardiac markers and infectious diseases.
Lateral flow tests are a form of immunoassay in which the test sample flows along a solid substrate via capillary action. After the sample is applied to the test strip, it encounters a colored reagent, which mixes with the sample and transits the substrate encountering lines or zones which have been pretreated with an antibody or antigen. Depending upon the analytes present in the sample, the colored reagent can become bound at the test line or zone. Recent improvements in lateral-flow testing have been made using both fluorescent and magnetic labels. Lateral flow tests can operate as either competitive or sandwich assays. Although their use has become widespread, lateral-flow tests have sensitivity limitations and tend to be semi-quantitative.
Researchers have developed the technology for analyte quantification (See for example, Rylant, D.; Moss, D.; Jane, A.; Bundesen, P., WO97/09620, 1997 and Polito, A.; Thayer, R. M.; DiNello, R. K.; Sierra, G. H.; Nixon, D.; Phillips, A.; Neubarth, S. U.S. Pat. No. 6,136,610, 2000). A number of platforms have been pursued using different particles and various detection techniques. Colored particles, quantum dots, fluorescent latex particles, liposome-based probes, magnetic particles, and Raman-active tags. Each of these detection techniques has its advantages and shortcomings. Absorbance based detection techniques often lack desirable detection sensitivity, while fluorescence techniques often require complex and expensive instrumentation. Lateral flow devices for magnetic-field measurements cannot be sealed inside a plastic housing, making it very difficult to use in POC markets. Like conventional fluorescence, Raman-based detection technology also needs expensive instruments.
Although, the current lateral flow test technologies are very useful because they are easy to use, there are many drawbacks related to current (or traditional) lateral flow tests. Most importantly, the current LFIA testing sensitivity is limited. In the lateral flow assay, gold particles, dyes or latex beads are used. These additional labeling, conjugation, and optical sensing steps are complicated which increase the manufacturing cost and testing cost
Sensitive systems for detecting biomarkers, known as biosensors, are valuable for early detection of initial onset of disease, or for monitoring post-treatment monitoring to detect recurrence of diseases. Biosensors, which can convert chemical or biological events into measurable signals, have two key components: 1) the recognition component and 2) the transducing/reporting component. The recognition component is responsible for carrying out specific interactions with the target molecules while the transducing/reporting component is responsible to convert the changes caused by the binding of target molecules into a signal that can be recorded. Such a signal can be mechanical, optical, electrochemical or electrical.